(1) Field of the Invention
The present invention relates to a material testing method for polymers bonded to metals and, more particularly, to a test for cathodic delamination of such materials in water.
(2) Description of the Prior Art
Many of the metal-polymer bond failures that occur in the marine environment are caused by a process referred to as “cathodic delamination.” Cathodic delamination is believed to result from the following reaction on a cathodically polarized surface exposed to seawater:2H2O+O2+4e−→4OH−  (1)
FIGS. 1 and 2 show a diagram of cathodic delamination. A material 10 is shown having a cathodically polarized metal substrate 12 with a polymer coating 14. Substrate 12 can be any cathodically polarized metal. Polymer coating 14 can be any polymer coating such as an encapsulant, paint or any other material that adheres to the surface of substrate 12. Material 10 is positioned in an aqueous environment 16.
The standard model for cathodic delamination proceeds as follows: water and dissolved oxygen in aqueous environment 16 diffuse through the polymer layer 14 and reach the underlying cathodically polarized metal surface 12. (Cathodic polarization is a commonly used method for imparting corrosion resistance to otherwise susceptible metals). Once there, the water and oxygen react with each other and with electrons 18 from the metal 12 to generate hydroxide ions at the metal/polymer interface 20. Once hydroxide ions are formed, additional water diffuses to the interface region because of osmotic pressure. In effect, the incoming water is trying to eliminate the concentration gradient for hydroxide ion between the metal/polymer interface region (pH≈14) and the surrounding water (pH≈8). The osmotic pressure differential causes formation of pressurized water-filled “blisters” 22 along the metal-polymer interface 20. When the internal pressure within the blister 22 exceeds the bond strength between the metal and polymer, the bond ruptures in that region.
Repeated millions of times at a very small scale, this process greatly weakens, and eventually eliminates, the bond between the metal and the polymer. If the polymer in question is paint, then the paint will blister and flake off exposing the underlying metal. If the polymer is an encapsulant protecting underlying electrical circuitry or devices, then catastrophic failure can result from exposure of vulnerable parts to seawater. Cathodic delamination is thought to be the major cause of polymer-metal bond failures in the marine environment. The cost of this process to the Navy is on the order of tens to hundreds of millions of dollars per year.
Commercial maritime interests and the Navy are keenly interested in designing and utilizing hardware that is as resistant to cathodic delamination as possible. One of the most commonly employed methods for determining cathodic delamination resistance is the saltwater accelerated life test (ALT). Such a test, if set up and run properly, allows an item of hardware to be artificially aged at a much faster rate in the laboratory than in real life. This allows the test engineer to predict the useful service life of the item under conditions conducive to cathodic delamination (i.e., “worst-case” scenario). Accelerated life tests can also be used to compare how well different materials resist cathodic delamination.
An accelerated life test is typically designed in accordance with the following equation:
                    RAF        =                  ⅇ                                    -                                                E                  a                                ⁡                                  (                                                            T                      2                                        -                                          T                      1                                                        )                                                                    R              ⁡                              (                                                      T                    2                                    ⁢                                      T                    1                                                  )                                                                        (        2        )            In this equation, “RAF” is the reaction acceleration factor; “e” is the base of the natural logarithm system; “Ea” is the activation energy for the process; “R” is the gas constant; “T2” is the temperature at which the accelerated life test is run; and “T1” is the normal operational environmental temperature for the item in question. Usually T2 is greater than T1 because the speed of most chemical reactions increases with temperature. Thus, by increasing the temperature in the accelerated life test, the rate of the cathodic delamination process can be sped up many-fold, allowing a relatively short duration laboratory test to simulate the equivalent of many years of exposure to normal conditions. The extent to which the higher temperature, T2, has sped up the reaction is indicated by the reaction acceleration factor. For example, if the reaction acceleration factor for a given accelerated life test set up is calculated to be “12”, that means the reaction will proceed twelve times faster at T2 than at T1. Thus, one month of exposure to the test conditions at T2 is equivalent to twelve months of exposure at T1. The key variable for determination of the reaction acceleration factor for a given set of temperatures is the activation energy, Ea, for the degradative reaction of interest. Ea is akin to an energy barrier that must be surmounted by reactants before a reaction can take place. The higher the Ea value, the slower (in general) the reaction proceeds at room temperature. Unfortunately, Ea is often unknown for a given material or set of materials and, therefore it must be determined experimentally.
Because water and oxygen need to diffuse through the polymer layer to trigger cathodic delamination under the standard model, all standard cathodic delamination accelerated life tests assume the Ea of interest is that for the diffusion of water into the polymer. This view is also re-enforced by the fact that, according to the prevailing model, water must continue to diffuse through the polymer and build up pressure in the interfacial blisters to trigger the actual delamination or debonding. This Ea value is typically calculated by first measuring water diffusion constants at three different temperatures for the polymer in question, and then relating them to the Arrhenius equation (the equation from which equation (2) is derived). One problem that develops concerns the numerical value of Ea. If it is low, the reaction in question occurs readily at temperature T1, and the reaction acceleration factor calculated using reasonable values for T2 is small. It should be noted that as T2 is increased, reactions that occur very slowly (if at all) at T1 can be accelerated to such an extent that they have significant effects on the materials being tested. This process can lead to erroneous conclusions from an accelerated life test, because if one is not careful, the observed deterioration may be the result of a process or reaction that occurs only at high temperatures (and thus, would not affect the materials of interest under normal environmental conditions). The problem is conducting a meaningful accelerated life test on a material with a low Ea for water diffusion and thus, a low reaction acceleration factor.